1. Field of the Invention
This invention relates to a method and apparatus for communicating digital information words and, more particularly, to an encoding/decoding technique wherein plural information words are encoded in an error-correcting code having a high error correcting ability so as to recover the original information words after transmission, recording, and the like.
2. Related Documents
The following documents describe encoding techniques which are helpful in providing background information and understanding to the present invention:
U.S. Pat. No. 4,189,710
U.S. application Ser. No. 30,652, filed Apr. 16, 1979 and now U.S. Pat. No. 4,238,852
U.S. application Ser. No. 31,030, filed Apr. 18, 1979 and now abandoned
U.S. application Ser. No. 86,677, filed Oct. 19, 1979 and now U.S. Pat. No. 4,306,305
"Optimal Rectangular Code for High Density Magnetic Tapes" by Patel and Hong, IBM Journal of Research and Development, pages 579-588, November 1974.
3. Description of the Prior Art
There are many applications for the transmission and recording of digital data. In one application, an audio analog signal is converted into a digital signal, and the digital signal is recorded directly, such as by a rotary-head type video tape recorder (VTR) or by a fixed-head recorder having a high recording density. The digitized audio signal generally is represented by a pulse code modulated (PCM) signal, and the technique of recording such digitized audio signals either by rotary-head type VTR's or by fixed-head recorders is known as PCM recording.
Although PCM recorders offer the advantage of high accuracy such that an original audio signal can be faithfully reproduced, PCM recorders, and digital data transmission systems in general, suffer from the drawback that noise, interference, signal dropout, and the like may be present either in the transmission link, in the transmitter (recorder) or in the receiver (reproducer) so as to destroy individual PCM signals. Such loss of data may result in serious errors in the reproduced signal so as to interfere with, for example, the reproduced audio signal. In an effort to minimize this problem, error-correction codes have been proposed, whereby the PCM signals (or other data information words) are encoded in such error-correction codes to allow for the correction or compensation of erroneous words at the data receiver (or recorder). If the data words contain errors which exceed the correction ability of the error-correction code, then such errors are compensated by replacing those erroneous words with words that are approximated from the correct data words.
Typical error-correction encoding techniques often are not successful in recovering digital information which is edited on a recording medium. For example, if PCM signals are recorded in longitudinal tracks on a magnetic tape, editing may occur either by splicing two tapes (e.g. tape A and tape B) together or by electronic editing (e.g. by replacing original PCM signals with new PCM signals from an edit point onward). Generally, when the information which is recorded on the magnetic tape is reproduced therefrom, multiple errors occur within the vicinity of the edit point (either the spliced edit point or the electronic edit point) which may exceed the error correcting ability of the error-correction code in which the PCM signals are encoded. Such multiple errors are known as burst errors, wherein a "burst" of PCM signals on either side of the edit point are distorted or otherwise made erroneous. Such burst errors appear as a multiple of consecutive, erroneous data words during reproduction. To minimize the effect of such burst errors, an interleave technique has been proposed in which, for example, data words that are out-of-sequence from each other are assembled, or interleaved, so as to form a data block. This data block may be formed of, for example, words #1, #47, #68, #125, . . . and the like. Even if multiple words in such a data block are distorted by burst error, when these words are de-interleaved during reproduction, such burst errors are sufficiently dispersed so as to appear as random errors. Thus, data which may have constituted an original block and which was interleaved to form a data block for recording purposes is only partially lost when that data block is de-interleaved during reproduction. However, even if erroneous words are dispersed, sufficient numbers of such words may be erroneous so as to exceed the error correcting ability of the error-correction code, thereby making satisfactory error correction practically impossible.
Yet another technique which has been proposed for avoiding burst errors which may be due to, for example, an editing operation, is to record a single channel of data information in multiple tracks so as to lower the probability that all of the data words in a single data block will be simultaneously erroneous. For example, PCM signals and an error correcting (or detecting) code signal may constitute a single error-correcting code block, this block being distributed in parallel, multiple tracks for recording. One type of code may be the optimal rectangular code (ORC). Thus, an error-correcting code block may be recorded as multiple words, each word being recorded in a separate track, these words being in alignment across the tape. However, with this multi-track technique, it is possible that, in the vicinity of the edit point, that is, in the multi-error section, a relatively high number of words in the error-correcting code block may be erroneous, thereby defeating error correction and error compensation.
A still further proposal directed to the recovery of data in a multi-error section of magnetic tape is described in German Patent Document No. 2,916,973. In this proposal, the data words which are reproduced from the magnetic tape are written into a memory and then, subsequently, are read out from that memory. When data is reproduced from the multi-error section of the magnetic tape, such data is not stored in the memory. This is because, since such data is expected to contain a high number of erroneous words, the storage of such erroneous words is avoided. As a result, the information which had been recorded in the multi-error section of the magnetic tape is not reproduced. To minimize this loss of data, the correct words which are reproduced prior to the multi-error section and the correct data words which are reproduced following the multi-error section are, effectively, joined such that the reproduced signals effectively "skip over" the multi-error section. To assist this joining of correct data words, the speed at which the magnetic tape is driven is increased from its normal playback speed when the multi-error section is reached. However, such change in the tape speed requires a synchronous change in the frequence of the clock signals which are used to store and retrieve the correct data words, as well as those clock signals which are used to process the reproduced data. Suitable control over the clock signal generator and timing circuitry is quite complicated. Also, if a time code is recorded on the magnetic tape, this technique is quite difficult to implement when the time code is used as a basis for the clock signals.
Yet another error-correcting encoding technique which has been proposed for a stationary-head PCM recorder is the so-called double recording technique. In double recording, each data word is represented as a main word and an identical sub-word. The main and sub-words are recorded in parallel tracks. In one modification of the double recording technique, the identical main and sub-words are time displaced with respect to each other so as to shift their respective recording positions. Thus, main data word #1 may be recorded in alignment with sub-word #55. Alternatively, the time-displaced main and sub-words may be interleaved and recorded in a common track. When these double recorded words are reproduced, if a main data word is detected as being erroneous, it is replaced by its corresponding sub-word. It is expected that, by time-displacing the main and sub-words, multiple errors which may occur in the multi-error section of the recording tape that might affect the main data word sequence would not affect its corresponding sub-word sequence. Thus, in the multi-error section, erroneous main data words are replaced by correct sub-words, the latter being recorded in a region that is sufficiently removed from the multi-error section. However, the implementation of this double recording technique is accompanied by high redundancy, primarily because each data word must be recorded in duplicate, and also requires relatively complicated apparatus. Furthermore, the main and sub-words generally do not undergo independent error-correction encoding and, therefore, the error correction ability of the double recording technique is reduced, especially in the vicinity of the edit point. Still further, if an erroneous main data word which is reproduced from the multi-error section is replaced by its corresponding sub-word, and if that sub-word is subjected to noise, drop-out, or the like, error correction and compensation is not possible.